I could think of 2 options:
- the drive was disconnected without having been undocked. You should run checkdisk and if necessary repair partition table next time drive appears.
- Sometimes improper cabling/power supply to external drive can have those effects
If you had that drive without a problem before and never disconnected improperly, chances are the controler (which makes the conversion from usb to sata) has a problem (or the drive in the enclosure has moved and has a bad connection). That is (with broken psu) most often cause a drive does not work properly. I do not believe your data to be at great risk.